1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor, and a method of manufacturing the capacitor.
2. Description of the Related Art
In general, a so-called multilayer ceramic capacitor is manufactured, when dielectric ceramic green sheets prepared beforehand from, for example, an Ni-containing paste constituting an inner electrode layer, and a dielectric layer forming paste constituting a dielectric layer are alternately laminated, and thereafter fired. During the laminating, since there is a demand for a thin multilayered configuration in order to increase electrostatic capacity at an equal volume, there has been a demand for further reduction of thicknesses of an inner electrode layer and a dielectric layer.
During the firing of this laminate, in general, since a dielectric layer forming material (ceramic material) and an inner electrode layer forming material (e.g., Ni) having different firing temperatures are simultaneously fired, a firing temperature is directly raised at a sintering temperature of the dielectric layer forming material.
However, when the inner electrode layer forming material and the dielectric layer forming material are simultaneously fired in this method, the inner electrode layer forming material having the firing temperature lower than that of the dielectric layer forming material is rapidly and excessively sintered at a high firing temperature, and an inner electrode is interrupted by so-called spheroidizing. That is, an infinite number of holes are made every positions in a layered member to be formed into a sheet member, a so-called worm-eaten sheet member (inner electrode layer) is formed. Furthermore, the spheroidizing of the inner electrode is said to induce delamination, and the dielectric layer peels off from the inner electrode layer.
When this spheroidizing occurs, an effective area (electrode coverage) of the inner electrode drops, and the electrostatic capacity of the capacitor is lowered. Therefore, in the present situation, the capacitor needs to be designed in consideration of the drop of the effective area by the interruption of the inner electrode. The drop of the effective area of the inner electrode requires the number of layers to such an extent that the drop is compensated, and this hinders miniaturization and large electrostatic capacity.
To handle this problem, in Japanese Patent Application Laid-Open No. 11-354374, it has been proposed that to form the inner electrode, specifications of the conductive paste be set in such a manner that the paste contains metal powder, and ceramic powder having an average particle diameter of ½ or less of that of the metal powder, and the ceramic powder occupies 2 to 40% by weight of a total solid content. This conductive paste is fired to form the electrode. However, in this proposal, the electrode and dielectric layer are fired under normal conditions, that is, at a firing temperature of 1240° C. in an non-oxidizing atmosphere. Therefore, the ceramic powder cannot remain in the inner electrode layer during or after the firing, and a function of sufficiently preventing the drop of the effective area of the inner electrode cannot be exerted. That is, in Patent Document 1 described above, the ceramic powder in the inner electrode forming conductive paste is gradually discharged on the side of a ceramic layer (dielectric layer) (paragraph [0022]).
Moreover, in Japanese Patent Application Laid-Open No. 2000-232032, particles for forming the inner electrode be used as an integrated particulate material of nickel and titanate to prevent a contraction difference accompanying the sintering of the dielectric layer and inner electrode. However, even in this case, it is said that the drop of the electrode effective area cannot be sufficiently prevented on normally performed firing conditions. An extra step of stably manufacturing the integrated particulate material of nickel and titanate is also required.
Furthermore, it is proposed in Japanese Patent Application Laid-Open Nos. 11-124602 and 2002-348603 that Ni metal particles be coated with a material having a high sintering temperature. Accordingly, a sintering start temperature is shifted toward a high temperature side, the temperature is brought close to the sintering temperature of the dielectric layer, and the electrode is inhibited from being spheroidized. However, in this proposal, even when the sintering temperature of the inner electrode can be shifted toward the high temperature, the material applied/added for the shifting toward the high temperature is diffuses into the dielectric layer at the time of the sintering of the dielectric layer. A problem occurs that composition of the dielectric layer changes, and original characteristics cannot be obtained especially in a case where the dielectric layer is thin. Even when the material applied/added for the shifting toward the high temperature has the same composition as that of the dielectric layer, the material diffuses toward the dielectric layer at the time of the sintering of a dielectric article. A problem remains that the compactness of the inner electrode layer is lost, and the inner electrode is interrupted.
The present invention has been developed under this situation, and an object is to provide a multilayer ceramic capacitor capable of inhibiting spheroidizing during forming of an inner electrode layer to prevent the inner electrode layer from being interrupted, that is, preventing drop of effective area of an inner electrode to obtain a high electrostatic capacity.